1. Field of the Invention
The invention relates to a capacitive fingerprint sensor, and more particularly to a capacitive fingerprint sensor which operates on a principle of charge-sharing.
2. Description of the Related Art
There are many known techniques of identifying an individual through the identification of the individual's fingerprint. The use of an ink pad and the direct transfer of ink by the thumb or finger from the ink pad to a recording card is the standard way of making this identification. Then, an optical scanner scans the recording card to get an image, which is then compared to fingerprint images in the computer database. However, the most serious drawback of the above-mentioned method is that the fingerprint identification cannot be processed in real-time, and thus cannot satisfy the requirement of real-time authentication, such as network authentication, e-business, portable electrical products, personal ID card, security system, and the like.
The method for reading a fingerprint in real-time has become the important technology in the biometrics market. The conventional method for reading a fingerprint in real-time belongs to an optical method, which is disclosed in, for example, U.S. Pat. Nos. 4,053,228 and 4,340,300. However, fingerprint sensors utilizing the methods may disadvantageously have large sizes and tend to be tricked by a fake image.
Consequently, chip fingerprint sensors, which overcome the drawbacks of the optical sensor and are formed by silicon semiconductor technology, are developed. According to the consideration of silicon integrated circuit (IC) processes, the capacitive fingerprint sensor has become the most direct and simple product. Tsikos discloses, in U.S. Pat. No. 4,353,056, a capacitive fingerprint sensor including capacitive sensing members arranged in a two-dimensional (2D) array, wherein capacitors formed between the plate electrodes and finger ridges are used to detect the fingerprint. However, Tsikos's method for utilizing an external circuit to sequentially scan each capacitor tends to be influenced by the wire's parasitic capacitance, and better image quality cannot be obtained accordingly. Knapp discloses substantially the same design in U.S. Pat. No. 5,325,442 except for one difference. Instead of sequential scan in Tsikos's patent, Knapp utilizes a single thin film transistor switch to control each capacitive sensing member, and the capacitors are charged by current so that the signals can be read. The capacitance differences between the capacitive sensing members can be obtained by measuring different charging currents. However, the problem of the wire's parasitic capacitance still cannot be effectively solved. Dickinson et al. disclose, in U.S. Pat. Nos. 6,016,355 and 6,049,620, a method for discharging capacitors in constant voltage, and the manufacturing processes of silicon ICs are used. Each capacitive sensing member utilizes a charge/discharge switch composed of a plurality of MOS transistors. First, each capacitor is charged to a constant voltage. Next, an external circuit controls a constant discharging current within a fixed time interval, the capacitance can be obtained by measuring the after-discharged voltage, and the 2D-image reading of the capacitance can thus be completed.
In the reading method by charging/discharging, however, the drawback resides in that the discharge MOS switches (controlled by external current mirrors) of each capacitive sensing member of the sensor must have uniform properties. Since a typical fingerprint sensor may include ten thousands of sensing members, it is difficult to control the uniformity of each discharge MOS in the manufacturing processes, and the uniformity of the output fingerprint signals is deteriorated. Although a larger discharge current may enhance the sensitivity of the sensing member, the power consumption thereof may also be increased. In a small discharge current, enlarging the time interval is needed to obtain enough sensitivity, the image acquiring speed has to be sacrificed.